Synthetic rutile is recovered as a TiO.sub.2 concentrate from TiO.sub.2 -containing substances, such as ilmentite and pseudorutile.
British Patent Specification 980,864 discloses a process in which ilmenite is reduced at temperatures from 1000.degree. to 1150.degree. C. with an addition of carbon, the reduced ore is separated by sieving and magnetic separation from the gangue and surplus reducing agent, and the metallic iron is oxidized by an oxidation effected in an acid aqueous medium having a pH value of at least 4 but below 7 and is thus liberated from the reduced ilmenite.
The oxidation is effected with agitation and with a supply of oxygen-containing gases into the suspension, either under atmospheric pressure and at a temperature in the range of 60 to 80.degree. C., or under a pressure of about 7 bars and at a temperature in the range from 150.degree. to 170.degree. C.
The oxygen-containing gas which is employed preferably consists of a mixture of air and CO.sub.2. Mixtures of oxygen and CO.sub.2, SO.sub.2 and NO.sub.2 are also mentioned as well as mixtures of SO.sub.2 and/or NO.sub.2 with air, oxygen and CO.sub.2 individually or in combination. An oxidation under a pressure of 7 bars and with a supply of air or of mixtures of oxygen with CO.sub.2, SO.sub.2 or NO.sub.2 will result in TiO.sub.2 concentrates having poor properties.
In the practice of that process the oxidation is effected under atmospheric pressure and with a supply of air and an addition of NH.sub.4 Cl. As that oxidation requires a long reaction time, that stage must be effected discontinuously and is highly expensive. The composition of the iron oxides cannot be controlled. If the ilmenite contains manganese, that process will require an afterleaching with sulfuric acid in a separate stage. The resulting acid filtrate must be subjected to a sewage treatment.